I've even gone into my yahoo account and signed out of everything to give that a try and not difference.
other notes. the account i'm trying to sign in with is also my account i use for windows login and is digitally signed with the same account. so unsure if that's a related some how to this issue.
if there is a fix for this else where, sorry, i haven't found it as I've only found post's about it since windows 10 was out, and recent that were all locked and seems like all of them were never resolved and re-installing windows should NOT be the answer to this... means something needs to be fixed with windows or the app or something. sadly, i'm not the only one who's had issues with this in the past, i just get it a lot because i reformat every so often.
i will have a screen shot of the error window that shows up EVERY time i try to add my yahoo account, BUT with every attempt, my email also get's a email saying it signed in...